Local electronic structure of Fe²⁺ impurities in MgO thin films: Temperature-dependent soft x-ray absorption spectroscopy study

We report on the local electronic structure of Fe impurities in MgO thin films. Using soft x-ray absorption spectroscopy, we verified that the Fe impurities are all in the 2+ valence state. The fine details in the line shape of the Fe 𝐿2,3 edges provide direct evidence for the presence of a dynamical Jahn-Teller distortion. We are able to determine the magnitude of the effective 𝐷4⁢ℎ crystal field energies. We also observed a strong temperature dependence in the spectra which we can attribute to the thermal population of low-lying excited states that are present due to the spin-orbit coupling in the Fe 3⁢𝑑. Using this Fe2+ impurity system as an example, we show that an accurate measurement of the orbital moment in Fe3⁢O4 will provide a direct estimate for the effective local low-symmetry crystal fields on the Fe2+ sites, important for the theoretical modeling of the formation of orbital ordering.